(1.) Field of the Invention
The present invention relates to a light-receiving amplifier circuit and an optical pick-up device using the same, and in particular to a technology for realizing a small-scale light-receiving amplifier circuit.
(2.) Description of the Related Art
In recent years, optical disc media (hereinafter simply referred to as “media”) such as a Compact Disk (CD), a Digital Versatile Disk (DVD), and the like, are widely used for recording large-volume digital information as represented by video and audio. As has already been known, laser beams of different wavelengths are used for reading and writing information into such various types of medium, depending on the type of medium.
A conventional optical pick-up device that is compliant with both CDs and DVDs is typically provided with a double wavelength semiconductor laser element serving as a light source, and a single optical system to be commonly used by two types of wavelengths. Through the optical system, the optical pick-up device amplifies an electric signal and outputs the amplified signal. The electric signal is obtained by performing photoelectric conversion, using light-receiving elements provided for the respective wavelengths, on a reflected light from media, projected onto a position that differs depending on the wavelength in accordance with a distance from a projecting point of the laser of the respective wavelengths.
As a light-receiving amplifier element suitable for such an optical pick-up device, a light-receiving amplifier circuit in which a light-receiving element and a differential amplifier which amplifies a signal of the light receiving element are provided for the respective wavelengths, and an output amplifier which selectively amplifies one of the outputs from the differential amplifier is known (e.g. see FIGS. 3 and 4, Japanese Laid-Open Patent Application No. 2004-22051).
The structure disclosed in Japanese Laid-Open Patent Application No. 2001-202646 is an example of further downsizing of a light-receiving amplifier circuit. According to this structure, light-receiving elements are separately placed according to different wavelengths, input transistors are set in an input stage of the differential amplifier for the number equal to the number of the light-receiving elements, and a switch is placed between each of the input transistors and each of the light-receiving elements.
The following describes a detailed structure and operation of the conventional light-receiving amplifier circuit disclosed in the above-mentioned Japanese Laid-Open Patent Application No. 2001-202646.
FIG. 1 is a diagram showing the configuration of the conventional light-receiving amplifier circuit. The light-receiving amplifier circuit includes switch elements 1-1, 1-2, 1-3 and 1-4 each of which uses a CMOS transistor, a differential input amplifier 2, two input transistors 2-2 and 2-3 which are placed in parallel on one of the differential inputs, and photodiodes (hereinafter referred to as “PDs”) 3 and 4. The PD 3 is connected to a base of the input transistor 2-2 while one end of the switch element 1-1 and one end of the switch element 1-3 are connected to that connection. Likewise, the PD4 is connected to a base of the input transistor 2-3 while one end of the switch element 1-2 and one end of the switch element 1-4 are connected to that connection.
The other ends of the switch elements 1-3 and 1-4 are commonly connected to a gain resistor 2-1. The other ends of the switch elements 1-1, 1-2 are connected to an appropriate potential (ground in this circuit example) so that the input transistor 2-2 is cut-off when the switch element 1-1 is ON and the input transistor 2-3 is cut-off when the switch element 1-2 is ON.
In the case of selecting a DVD photodiode 3, the switch elements 1-1 and 1-4 are turned off and the switch elements 1-2 and 1-3 are turned on, so that the paths from the PD 3 to the gain resistor 2-1 and the input transistor 2-2 respectively are made valid. In contrast, in the case of selecting a CD photodiode 4, the switch elements 1-1 and 1-4 are turned on and the switch elements 1-2 and 1-3 are turned off, so that the paths from the PD4 to the gain resistor 2-1 and the input transistor 2-3 respectively are made valid.
Since no resistors are placed in series between the PD3 and the input transistor 2-2 as well as between the PD4 and the input transistor 2-3, it is possible to switch between the photodiodes without degrading the performance originally provided in an amplifier.